Radio frequency transmissions are often subjected to multipath fading. Signal blockages at receivers can occur due to physical obstructions between a transmitter and the receiver or service outages. For example, mobile receivers encounter physical obstructions when they pass through tunnels or travel near buildings or trees that impede line of sight (LOS) signal reception. Service outages can occur, on the other hand, when noise or cancellations of multipath signal reflections are sufficiently high with respect to the desired signal.
Communication systems can incorporate two or more transmission channels for transmitting the same program or data to mitigate the undesirable effects of fading or multipath. For example, a time diversity communication system delays the transmission of program material on one transmission channel by a selected time interval with respect to the transmission of the same program material on a second transmission channel. The duration of the time interval is determined by the duration of the service outage to be avoided. The non-delayed channel is delayed at the receiver so that the two channels can be combined, or the program material in the two channels selected, via receiver circuitry. One such time diversity system is a digital broadcast system (DBS) employing two satellite transmission channels.
Interleaving of data symbols on the transmission channels of a time diversity system can be employed to mitigate, in particular, the effects of a slow deep fade. An interleaver rearranges a set of consecutive coded data symbols in a data stream to be transmitted such that symbols in the set extend for a duration of time greater than the duration of a slow deep fade. A receiver having a de-interleaver rearranges received symbols to their original order. The interleaved data symbols that are being de-interleaved, however, are subject to independent fades that may not be mitigated by the interleaver due to size constraints of the interleaver.
In addition, a DBS generally has a requirement for protection against an outage of selected minimum duration when both satellite channels are available. Thus, a need exists for a communication system which provides such outage protection. In addition, a need exists for a communication system which provides maximum outage protection within reasonable memory and delay constraints of the interleaver when only a single satellite channel is available.